Method of grinding



DGC. G, 1938. J v SLADE Re.'20,941-

METHOD OF GRINDING Original Filed Oct. 14, 19752 3 Sheets-Sheet 1mvEN'roR JAMES v. SLADE ATTORNEY 1938. J. v. SLADE Re. 20,941

METHOD OF GRINDING Ori ginal Filed Oct. 14, 1932 3 Sheets-Sheet 2INVENTOR 'JAMES V. 51.40.:

ATTORNEY Dec; 6, 1938.

.1. v. ESLADE METHOD GRINDING 3 Sheets-Sheet 3 Original Filed Oct. 14,1952 7'0 THICKENER TDTH/CKENER INVENTOR.

JAMES nsuos BY ATTORNEY Reissued Dec. 6, 1938 PATENT OFFICE METHOD OFGRINDING James V. Slade, New Rochelle, N. Y., assignor to The DorrCompany, Inc., New York, N. Y., a

corporation of Delaware Original No. 2,052,426, dated August 25, 1936,Se-

rial No. 637,696, October 14, 1932.

Application for reissue August 23, 1938, Serial No. 226,309

16 Claims.

This invention relates to improvements in grinding systems, moreparticularly the two stage closed circuitsystem which is more or lessstandard in the metallurgical industry and also used in the non-metallicmineral industry, for instance in the cement industry.

One object of this invention is to provide greater flexibility ofoperation in a closed circuit two stage grinding system. Morespecifically this means rendering more positive and flexible, the

operation of the primary classifier, which heretofore has labored undercertain shortcomings.

Other objects are to simplify the plant layout and to eliminate certainheretofore bothersome l5 restrictions in the way of mechanicalarrangements, and to increase the degree of efiiciency and utilizationof existing machinery.

These and other advantages are obtained by a change in the flow sheet orhook up, featuring essentially the return of part or all of thesecondary mill discharge to the primary classifier. The nature of thisarrangement will be better understood when it is recalled that astandard flow sheet for closed circuit grinding generally 25 includes aprimary and a secondary closed grinding circuit with each circuitincluding a bill and an associated classifier. Both circuits operate inseries, with the overflow from the primary classifier feeding into thesecondary circuit or see- 30 ondary classifier. The invention may alsobe said to consist in utilizing material obtained from one stage tocontrol and improve the operation of the preceding stage.

Standard equipment called for by this flow 5 sheet comprises a ball orrod mill for coarse grinding in the primary circuit, and another millfor fine grinding in the secondary circuit. For proper operation of thesystem the primary classifier must separate undersize particles up to acertain 43) desirable size, which may be called the intermediate size.Thisis the size of material which is transferred together with finersizes to the secondary cycle for fine grinding.

,That intermediate size is not always readily or positively obtainablein the ordinary run of rake classifiers owing to the fact that theseparation in the classifier depends upon a variety of factors such asrake speed, the amount of wash water used, the dilution of the pulp, andupon the area of the classifier bath from which the overflow orundersize is withdrawn. In other words the character of the separationdepends upon what in the pertinent terminology is called the overflowcapacity of the classifier, and

55 which may be defined as the tonnage of undersize solids up to acertain mesh size which the machine is capable of delivering free orsubstantially free from oversize.

Since the above factors have bearing upon each other and upon theclassifier operation, they can- 5 not be freely 'and individuallychanged for the purpose of controlling the overflow capacity withoutimposing other difficulties and limitations upon the classifieroperation.

The operator tries to correct such discrepancies 10 in various ways. Onetendency is to operate the classifier at increased speeds to try to keepthe bath better agitated to hasten its selective action which may leadto an otherwise undesirable excess of speed. Another tendency is toreduce the amount of liquid passing through the classifier to reduce thedilution, which increases the buoyancy action upon the intermediate sizeof solids, at which procedure he may run to a point at which properselectivity is lost. Moreover, while this reduction of liquid contentsproduces a bath of excessive specific gravity and viscosity, it alsoimposes certain limitations upon the amount of spray water allowable,which in turn results in a rake product of insufficient cleanliness.

The primary classifier is required to separate a desired undersize ofsolids in the overflow (intermediate plus finer sizes) for furthertreatment in the secondary circuit, and further to have sufficientraking capacity in order to handle the necessary amount of circulatingload. In other words the classifier must be selective in certain waysbut its selectivity must not be such that it makes its reaction feltupon other phases of the classifier operation. So there are twoantagonistic factors to be reconciled in this machine.

Still in other words the difliculty is that the demand for a certainraking capacity results in a greater width of the classifier which inturn influences the overflow capacity, tending to produce a much finerseparation than desired for intermediate size.

Still another factor may tend to render diflicult the proper coarseseparation in the primary classifier, namely a certain minimum amount ofcirculating liquid is required for the proper operation of the ballmill, and this required minimum liquid under some circumstances exceedsthe amount desirable for the separation of the intermediate size in theclassifier overflow. On the other handthis minimum amount of water mayhave to be augmented by extra water added to the mill feed if thedischarging rake product or sandsfrom the classifierkare too dry. In thelatfunction of the two classifiers should here be noted. Theprimaryclassifier is pressed into a service for causing the separationof fines plus certain solids of a desired intermediate size, with anattendant excess of overflow capacity. As this overflow enters the bowlof the secondary classifier it encounters a deliberately increasedoverflow capacity in the form of the enlarged circular overflow launderof the bowl, which produces the very fine separation desired. Thisoverflow constitutes the final product from this fiow sheet. Thematerial of intermediate size is raked out by the classifier andcirculated through the secondary. grinder or tube mill to be reduced tosaid final size, and then to be separated out upon its return to thebowl ofthe secondary classifier. The final slurry is usually dewateredin a thickener from "which the sludge may be further treated in a vacuumfilter.

The connection between two such circuits may be said to be a bottle neckof the operation. This means that it is difficult not only to adjusteach circuit for proper and emcient operation and cooperation of themachine units within itself, but it is also difficult to tune'upproperly the cooperation of the two circuits with each other,particularly in view of fluctuations in the amount and size of the freshfeed thereto. As a consequence of such fluctuations, one circuit may beoverloaded, as would appear from an excess of material loading down therakes, while the other circuit may operate below capacity. The dimcultytherefore, is found in a certain lack of flexibility within the system,and which is further I aggravated where each circuit, and the connectionbetween the circuitsrequires a pump for maintaining the transit of thematerial.

Pumps in such operation may form a source of trouble in that they aresubject to heavy wear and tear due to the abrasive and coarse characterof the charge. Moreover, irregularities in their operation may causesudden surges which are liable to upset certain desirable quiescent flowconditions, as for instance, in the sensitive feed to the bowl of thesecondary classifier. Also the character of some materials may precludethe use of pumps'altogether.

Difliculties are furthermore encountered inthe practical layout of suchplants, in that very careful consideration must be given to the relativeelevations and locations of the various machine units to effect theproper transit of the material.

Various kinds of mechanical elevators and special feedersmust frequentlybe resorted to in order to carry the material from one step of theoperation to the next, when the dilution of the material and amount ofcoarse material therein, as in the primary circuit, precludes the use ofotherwise convenient and desirable pump arrangements.

One main feature of this invention serving to overcomethe difflcultiesenumerated, lies in the fact that in a suitable manner an auxiliaryvolume of slurry is made available from the process and introduced intothe bath of the primary classifier, so that it can be said that theprimary classifier is operated with a tonnage in the overfiow which isgreater than the tonnage of new feed entering the primary cycle.

The invention thus comprises the control of return of slime valuesthereto to aid coarser separation in that step, but also the return ifdesirable of relatively coarse material from the secondary grinding step"to the primary cycle for the purpose of load regulation in the grindingstage.

Hence the embodiment herein shown provides for returning or bypassingmaterial discharge from the secondary mill, through the primaryclassifier instead of direct to the secondary classifier.

Furthermore, as a consequence of the above changes of operationthis'invention makes the use of pumps more generally possible and theiroperation positive, thereby displacing expensive elevators, and otherbulky and unhandy arrangements.

Numerous advantages as hereinafter described are the result of theimproved arrangement according to this invention.

The manner of carrying out this invention may be understood by referringto the accompanying drawings in which Figure l is a diagram showing thepresent improvement over a standard system. Figure 2 is the sideelevation, systematically shown of a practical arrangement of theimproved system. Figure 3 is a cross sectional view taken upon Figure 2,disclosing the comparati'vely large width of the primary classifier andthe distribution of the feed thereto. Figure 4 is a detail showing of amill unit including the primary and the secondary mill combined. Figure5 is a corresponding duplication of the showing of Figure 1, merelyillustrating the improvement in a somewhat difierent and moreconventional manner. Figure 6 shows a modification or compromise in thatonly a portion of the material from the secondary cycle is bypassedthrough the primary classifier or circuit.-

overflow capacity in the primary classifier by the The flow sheet of thestandard system includes is returned to'the primary feed, thus closingthe primary'grinding cycle. The returning material constitutes the socalled circulating load in closed circuit grinding;

The overflow from the primary classifier contains intermediate and finesized material, which is transferred into the secondary grinding cycle,by way of feedingit into the secondary classifier which latter accordingto standard practice isthe bowl type of classifier. In the secondaryclassifier, the intermediate sizeis separated through the rake dischargeand removed, and after passing through the secondary mill is circulatedback into the bowl feed, thus closing the secondary cycle. The overflowfrom the bowl con- *tains the fines which are delivered from the systemas a final product indicated in the form of a thin pulp or slurry.

The improved arrangement of this invention is shown in diagram numberedFigure 1. Instead of tying the two cycles or circuits together with afeed line between the primary and the secondary classifiers it is to benoted that in Figure 1 the primary and the secondary circuits merge intoeach other as both mill discharges in feed into the pool of the primaryclassifier, thus eliminating the usual bottle neck and attaining manyother advanta es explained hereinafter.

In Figure 1, fresh feed 2| enters the primary mill 22. Upon leaving themill as indicated at 22* the material is separated in the primaryclassifier 23 where the coarse solids are caused to leave through therake discharge in order to re-enter the primary mill, thus closing theprimary circuit, while intermediate sized particles and fines escape toenter the secondary circuit as they feed the secondary or bowlclassifier 24.

There again separation takes place, with inter-' mediate sized solidsleaving the rake discharge towards the secondary mill 25. The discharge25 from the secondary mill as indicated at 26 joins the primary milldischarge 22 in feeding the primary classifier 23. Final fine size asoverflows from the bowl leaves the secondary circuit as indicated at 21.

The arrangement in Figure 2 includes the mill unit 28 comprising aprimary and a secondary mill or compartments 29 and 3'3 respectively.Fresh feed 3| plus returned circulating load from a primary classifier32 enters a feed container 5' 33 from which a customary scoop or spiralfeeder 34 may feed it into the primary grinding compartment of the unit.Discharge from the primary and from the secondary compartment join in acommon discharge chamber 35 between the compartments and leaving themill in the direction of the-"arrows through openings 36 in the drum todrop into a sump 31 below the mill from which a ump 38 lifts it throughriser 39 to the feed thro h 40 of the primary classifier 32. The primary0 assifier has a customary washing spray 4| for th rake product, and iselevated substantially above the mill unit high enough to allow 5 therake product to gravitate at 42 'to theprimary mill' feed, and theprimary classifier overflow 43 to gravitate to feed a secondaryclassifier 44 which latter again is located high enough to allow itsrake productto gravitate at 45 to the secondary mill feed. The overflowfrom the bowl 46 of the secondary classifier constitutes the final rproduct to be delivered from the grinding system, as indicated at 41.

fresh feed is indicated in dot and dash at 48 where fresh feed may enterthe feed trough 4D 50 of the primary classifier 32.

Figure 4 shows more clearly and in detail a preferred embodiment inwhich the primary and the secondary mill are combined in one rotary ballmill .unit 49. The showing is intended to 5 convey merely the featurethat separate feeds enter at each end of the drum unit, and coma, binedleave a common central discharge chamber. The feeds enter for instanceby way of scoop members or feeders 50 and 5| through hollow trunnionsinto a primary and a secondary grinding compartment 52 and 53respectively of the drum unit. The ends of the drum are shown to besupported in bearings 54 and 55. A gear 56 surrounding the circumferenceof the drum indicates the drive. The primary compartment is indicated ashaving large sized grinding balls for coarsegrinding and the secondarycompartment as having smaller sized balls customary for fine grinding.At their inner ends the compartments are confined by partitions 51 and58 having central discharge openings 59 and from which the material isdelivered into a central discharge chamber BI and through peripherallyarranged openings 62 in the drum out into a sump 53.

It is noted that with this improvement the bottle neck is eliminated, inthat the two circuits merge directly into each other with'the bath orpool of the primary classifier forming the junction.

While the elimination of the bottle neck between the circuits is mostclearly illustrated in the largely diagrammatic showing of Figure 1, amore conventional showing of the fiowsheets has been given in Figure 5.These figures correspond in substance exactly to the Figure 1, butafford a more accurate showing ofthe relative position of the machineswith respect to thelocation of the respective inlets and outletsthereof.

Figure 5 illustrates the improvement upon the standard flow-sheet,analogous to Figure 1. The operation consists in feeding raw material atH to the primary mill 22, transferring the mill dis-' charge 'I2'to theprimary rake classifier 23, and allowing the rake product to gravitateback as at 13 to feed the mill, while the classifier undersize 14 goesto feed the bowl of the secondary classifier 24. Rake product 15 fromthe secondary classifier feeds the secondary or fine grinding mill 25,mill discharge 16 being returned over the primary classifier 23 to thesecondary classifier 24. Of course the transfer of material in thisflowsheet may be maintained by a single, pump handling the combineddischarges from both mills to feed the primary classifier, in a mannerpreviously described in connection with Figures 2 and 3 (but here notshown). Final size leaves the bowl overflow at 11.

Fig. 6 shows a modification, or compromise the material from the milldischarge is split off and returned to the primary classifier orcircuit. In other words, a portion of the material obtained from thesecondary mill discharge is bypassed through the primary classifier orcircuit, whereby it should be understoodthat'the split off portion maybe selected according to size characteristics, or quantity proportion,depending upon what the specific requirement of operation be. Figure 6is distinguished substantially only by the arrangement of a bypassconnection 18 between the sec- An alternative manner of introducing theondary mill discharge and, the primary classifier feed, branching off aportion of the secondary load at a splitting point indicated at 19.Final size leaves the system through bowl overflow at 1 80. Noparticular mechanism is indicated at this point, while it is to beunderstood that the split may be effected in-various suitable ways. Morebroadly, this alternative arrangement brings within the scope of thisinvention the idea that a selected portion of the material in the secfrom the secondary or tube ,mill to the primary classifier, thedifliculty of separation is largely relieved. This return of secondarymill discharge constitutes an auxiliary volume of slurry available tocreate the desired overflow intensity in the primary classifier, andpromotes the desired separation of the desired size of solids in theprimary classifier, in that it helps to lift, buoy up and flow away, thedesired intermediate sire, of particles. This beneficial effect, it willbe understood, is due to the large proportion of fines or slimes in thesecondary mill discharge. now produced a more intense flow over theweir, without excessive dilution due to the presence of the slimes.

Expressing this in terms of buoyancy, the properties of added slimesproduces sufllcient buoyant force to stimulate the separation. on theother hand, the amount of dilution may be balanced against a moreliberal supply of spray or washwater in the classifier whereby neitherthe thickness or viscosity of the bath nor, the rake speed need beexcessive, and a. clean rake product may be obtained. In other words,the slime values and added tonnage in the primary classifier 'permitrelatively higher dilutions for a given separation. Thus the operationof the primary classifier is rendered more flexible and free fromdisturbing factors.

While the present embodiment merely illus-* trates the return ofmaterial from the secondary mill discharge to the primary classifier, itshould be understood that the added slime value in the primaryclassifier may also be produced from various other points or sources ofthe treatment process. For instance it may be represented by arecirculated portion of the primaryclassifier itself, by the overflowfrom the secondary classifier, or else by the sludge recovered from afinal thickening step past the secondary clarifier, in which alternativecases it can be said that all the undersize returned to the primaryclassifier is finer than the critical mesh of separation in the primaryclassifier.

In the preferred embodiment of this invention, the secondary milldischarge together with the primary mill discharge is pumped to feed theprimary classifier. This. affords a three-fold advantage. First the needfor'two. circulating pumps (one for each circuit) in the old system, isre:- duced to only one, thereby reducing the first-or installed cost andsimplifying the plant layout as well as its operation. This really meansa saving of two pumps when considering that normally each pump requiresa spare pump for such emergencies as would be. expected when handlingmaterial diflicult to pump and having abrasive qualities. Second, thelubricating effect of the added slimes upon the coarse sands obtainedfrom the primary balimill greatly improve the mechanical operation andthe service of the pump proper. This means less wear together with morereliable and smoother running of the pump. Moreover it may mean thatonly thus is the use of a pump made possible at all. Third, theirregularities of the pump operation, manifesting themselves normally insudden surges orthe like, are now spread over and absorbed in the entiresystem. The importance of this point will be evident upon comparisonwith the old arrangement where individual pumps serve the primary andthe secondary circuits. Asurge in the pump of thesecondary circuit woulddirectly affect the sensitive overflow of the bowl of the secondaryclassifier, but with There is therefrom is absorbed or dispersed throughthe bath or pool in the primary classifier which is interposed betweenthe pump and the bowl. Furthermore, any possible surge is smoothed outthrough the otherwise improved operation of the pump itself. I

Due to the new arrangement as shown, the two closed circuits are nowmuch more intimately coupled with the efiect that changes in the newfeed are more readily distributed over both circuits, instead ofoverloading or choking one circult, and leaving the other more or lessidle. Through the medium of the combined mill feed to the primaryclassifier, the effects of such uneven loads are adjusted between thetwo circuits. To be more specific corresponding to a change orfluctuation in the new feed size, the separation of the intermediatesizein the primary classifier can be varied between the practical limits ofsay 8 and 65 mesh, whereby disregarding the original feed size, the loadcan be juggled between the primary and the secondary circuit so thatboth mills and both classifiers will be evenly taxed. .The control ofsuch separation can be readily efiected by a change in the amount ofspray or wash liquid in, or of feed water to, the primary classifier, orelse by a change in the classifier speed. In any case, the primaryclassifier forming the coupling between the two circuits will equalizeor absorb the shocks from feed and other fluctuations. Thus can beobtained the desired even load distribution between the primary and thesecondary mill which is vital for their efliciency, and the preservationof the grinding parts.

The flexibility in the system, giving opportunity for even loaddistribution between the primary and the secondary mill offers aspecific advantage in the shutting down operation of the mill. In caseof a shut-downfor overhauling or repair, it is desirable to remove fromthe system a large portion of the material normally in transit, beforestopping operation of the plant. After stopping the feed, the systemthen has to be allowed to run the circulating load down to a certainminifrom the load within a much shorter time because both mills areworking simultaneously upon the dwindling load. Both circuits can bekept in balance to exhaust the load in the classifier baths, whichavoids having the primary mill running empty while waiting for thesecondary mill to finish up.

correspondingly, this arrangement also helps the starting up of thecircuits, by permitting coarser temporary separation to bring up thesecondary or tube mill load evenly or concurrently with the primaryload, instead .of running very light until after the primary circuit hasbuilt up a heavy load. Thus, both circuits may start evenly and beloaded up quickly by temporarily increasing the feed. Considerablesavings of time can thus be attained, and the wear and tear ofgrindingparts in the mills be reduced.

Another practical consequence of great importhe primary and secondarymills are coupled as a unit. In this unit, the primary mill is bound tothe secondary mill for coaxial rotation, and it is therefore desirable,if not vital, that both mills shoulder be under a load while running.

Heretofore, such difficulty has been overcome only From the foregoing itcan be seen that the primary classifier according to the novelarrangement constitutes what may be called the coupling between theprimary and the secondary circuit, inasmuch as that classifier is closedcircuited through the flow of its oversize or rake product with theprimary mill, and on the other hand also closed circuited through itsundersize or overflow product with the secondary mill cycle. Furthermoreit can be seen that this classifier not only forms a cushion forrelative irregularities or fluctuations in the circuits, but also offersa point control of the system by manipulating the operation of theclassifier, furnishing means for shifting the bulk of circulating loadsback and forth between the stages.

While all these advantages arev derived from the relatively simplechange in the fiow sheet as explained there is inherent in the newhookup still another advafitage discernible when the introduction of thenew feed is slightlymodified. This modification relates to introducingthe new feed as at 48 in Figure 2 into the primary classifier feed whichaccording to this invention comprises the primary plus the secondarymill discharge. Introducing the new feed which may be dry, into thisample feed volume gives an opportunity to thoroughly wet the same, andfurthermore separating certain fines directly out of the new feed as itpasses through the primary classifier, prevents the fines which arecontained in some of the raw feeds in excess, from unnecessarily passingthrough the primary mill (into which ordinarily the new feed is firstintroduced).

According to one desirable arrangement, both mills may be located atsubstantially the same elevation, the mixture of both gravitating to apump by which it is lifted through a riser pipe to the primaryclassifier located at a suitable height elevation. The rake product fromthis classifier isallowed to gravitate to the primary mill feed where itmingles with the new feed. The elevation of the primary classifier ishigh enough, not only to allow unhampered gravitation of the coarse tothe mill feed, but also high enough to allow the classifier overflow tofeed the bowl of the secondary classifier which in turn may dischargeits rake product gravitationally into the secondary mill feed. Since thefeed to the primary classifier, according to the improved operation isof greater volume and contains a relatively larger proportion of fines,it will be therefore more easily and evenly distributed over theclassifier feed trough or inlet (see Figures 2 and 3) than washeretofore possible. This is particularly desirable where the classifieris of considerable width and capacity such as is often. called for wherehigh circulating loads are desired. The bowl overflow from the secondaryclassifier may feed a customary thickener (not shown) to dewater thefinal slurry.

The preferred embodiment also shows the use of a somewhat modifiedcompartment mill unit.

' A standard compartment mill ordinarily used for two stage straightgrinding has an arrangement for the separate disposal of each milldischarge including a partition between both mill compartments, leavingdischarge space on each side there of, separate outlet means'andseparate sump and pump arrangements for each discharge. This is nowreplaced by a single discharge chamber of reduced size between themills, eliminating the partition with a single sump below, and singlepump, making the whole a simple'and compact aggregate.

It will be readily seen that with pumping now rendered easy andreliable, the elevations and relative location of the various machineunits of the system can be more liberally chosen; various types ofmechanical feeders, elevators and the like can be eliminated; the milldesigner having less restrictions on him can make better use of, andadapt design more readily to given local conditions, at a reducedexpense of layout and machinery involved.

I claim:

1. A method for closed circuit two stage grinding which comprisescirculating new feed through a primary operating cycle including a wetcoarse grinding stage and a wet classifying stage, returning theclassified coarse product to the coarse grinding stage, advancing theintermediate product separated from said classifying stage into asecondary operating cycle including a wet finegrinding stage and anotherwet classifying stag".

ahead of said fine grinding stage, discharging the fines separated fromthe secondary classifying stage for utilization; characterized by thesteps of mixing discharge from the primary and from the secondarygrinding stages, and returning the mix ture to the primary classifyingstage.

2. In a closed circuit two stage grinding system, the combination with aprimary operating cycle including a coarse wet g'rindingmill, and a wetoperating classifier; a secondary operating cycle including a wet finegrinding mill, and a wet operating classifier; of pump means and conduitconnections therefor, arranged for transferring combined discharges fromboth mills to the primary classifier. i

3. A method for closed circuit two stage grinding which comprisescirculating new feed through a primary operating cycle including a wetcoarse grinding stage and a wet classifying stage, returning theclassified coarse product to the coarse grinding stage, advancing theintermediate product separated from said classifying stage into asecondary operating cycle including a wet fine grinding stage andanother wet classifying stage ahead of said fine grinding stage,discharging the fines separated from the secondary classifying stage forutilization; characterized by the step of pumping combined dischargefrom the primary and the secondary grinding stage into the primaryclassifying stage.

4. A method for closed circuit two stage grinding which comprisescirculating new feed through a primary operating cycle including a wetcoarse grinding stage and a wet classifying stage, retur-ning theclassified coarse product to the coarse grinding stage, advancing theintermediate product separated from said classifying stage into asecondary operating cycle including a wet fine grinding stage andanother wet classifying stage ahead of said fine grinding stage,discharging the fines separated from the secondary classifying stage forutilization; characterized by the step of pooling discharge from theprimary and the secondary grinding stage in the primary classifyingstage.

5. A method for closed circuit two stage grinding which comprisescirculating new feed through a primary operating cycle including a wetcoarse grinding stage and a wet classifying stage, returning theclassified coarse product to the coarse grinding stage, advancing theintermediate product separated from said classifying stage into asecondary operating cycle including a wet fine grinding stage andanother wet classifying'stage ahead of said fine grinding stage,discharging the fines separated from the secondary classifying stage forutilization; characterized by the fact that a mixture of discharge fromthe primary and from the secondary grinding stages is treated in theprimary classifying stage, and that new feed is introduced together withcirculating feed to the primary classifying stage.

6. A-method for closed circuit two stage grinding whichcomprisescirculating new feed through a primary operating cycleincluding a wet coarse grinding stage and a wet classifying stage,returning ,the classified coarse product to the coarse grinding stage,advancing the intermediate product separated from said classifying stageinto a secondary operating cycle including a wet' fine grinding stageand another wet classifying stage ahead of said fine grinding stage,discharging the fines separated from the secondary classifying stage forutilization;' characterized by the fact that a mixture of discharge fromthe primary and I from the secondary grinding'stages is treated in theprimary classifying stage, whereby combined fiow from both grindingstages is pumped to the classifying stage, and that the intermediateoverfiow separation is allowed to gravitate from the primary classifyingstage to the secondary classifying stage, and the secondary coarseseparation to gravitate to the secondary grinding stage.

7. A method for closed circuit two stage grinding which comprisescirculating new feed through a primary operating cycle including a wetcoarse grinding stage and a wet classifying stage, returning theclassified coarse product to the coarse grinding stage, advancing theintermediate prodnot separated from said classifying stage into asecondary operating cycle including a wet fine grinding stage andanother wet classifying stage ahead of said fine grinding stage,discharging the fines separated from the secondary classifying stageforutilization; characterized by the fact that a mixture of dischargefrom the primary and from the secondary grinding stages is treated inthe primary classifying stage.

8. A method for wet grinding in stagewise operation, in which anoperating stage comprises a step of wet grinding and a step of wetclassifying, said steps to operate in closed circuit relation to eachother with oversize material from the classifying step returning to thegrinding step, delivering a slurry containing undersize material fromsaid classifying step into another similar closed circuit operatingstage and which comprises another wet grinding step and another wetclassifying step, and reintroducing a controlled amount of the materialreground in the latter circuit into the classifying stageof thepreceding circuit.

9. A method for wet grinding in stagewise operation, in which anoperating stage comprises a step of wet grinding and a step of wetclassifying, said steps to operate in closed circuit relationto eachother with classified oversize material returning to the grinding step,delivering a slurry containing undersize material from said classifyingstep into another similar closed circuit operating stage comprisinganother wet grinding step and another wet classifying step, andreintroducing into the first mentioned circuit an amount of slurry intransit in the latter circuit, containing undersize produced by thelatter grinding step, said undersize in passing through the firstmentioned classifying step being effective to change the dilution orviscosity of the slurry therein and thereby consequently changing thebuoyancy of the slurry with respect to coarser particles, in order toeifect the grade of separation desired in said first mentionedclassifying step.

10. A method for wet grinding in stagewise operation, in which anoperating stage comprises' a step of wet grinding and a step of wetclassifying, said steps to operate in closed circuit relation to eachother with classified oversize material returning to the grinding step,delivering a slurry containing undersize material from said classifyingstep into another or later similar closed circuit operating stagecomprising another wet grinding step and another wet classifying step,withdrawing from the process a slurry containing fines produced in thelater circuit and reintroducing it into the earlier circuit, said finesin passing through the first mentioned classifying step being efl'ectiveto change the dilution or viscosity of the slurry therein and therebyconsequently increasing the buoyancy of the slurry with respect tocoarser particles, in order to aid a desired coarser separation in saidclassifying step.

11. A system for stage closed circuit grinding comprising a wet grindingstage and a wet classifying stage operating in a primary closed circuit,and another wet grinding stage for further comminution of undersizematerial obtained from the overflow of the first-mentioned classifyingstage and another wet classifying stage operating in circuit with saidsecond grinding stage characterized by means for combining milldischarge from both grinding stages, and conveying means for introducingsuch mixture into the primary classifying stage to effect therein aredistribution of the proper and desired size solids fromthe mixtureinto and over the respective grinding circuits.

12. A closed circuit two-stage grinding system according to claim 13 andhaving in said circuits a primary and a secondary classifierrespectively of the settled solids conveying or raking type with a rakeproduct discharge and an overfiow, and having correspondingly a primaryand a secondary mill, in which, system a pump lifts the combined milldischarges to the primary classifier located at an elevation above themill feed, in which are provided means for allowing gravitationaldischarge of the rake product of the primary .classiiier to feed therespective primary mill, in which are also provided conduit means forgravitational feed of the primary classifler overflow to the secondaryclassifier; and in which are provided conduit means for gravitationallyconveying secondary rake product to the feed of the secondary mill.

13. A system of stagewise closed circuit grinding which comprises aprimary and a secondary closed circuit'grinding arrangement for coarsegrinding and fine grinding respectively, each such primary circuit forcoarse grinding, another connection for delivering undersize or overflowto the classifier of the secondary circuit, said grinders beingstructurally and coaxially combined to form grinding sections of arotary drum unit and having interposed therebetween a discharge deliveryspace, each section having a feed arrangement for supplying respectivecirculating loads to the respective grinding sections, each section alsohaving a discharge arrangement for ground material, means for combiningthe respective,

materials discharged from said grinding sections, means for returningthe combined discharges to the classifier of the primary grindingarrangement, the size separation in said last mentioned classifier beingcontrollable by controlling the operation of the classifier to effect aload distribution with regard to the grinding stages to enable eachgrinding section to be kept under a desired load while the unit rotates,as by the shifting of load, for instance, from the secondary grindingcompartment to the primary grinding compartment of said rotary drumunit.

14. A method of stagewise closed circuit grinding in which the materialpasses through a primary and a secondary cycle, each cycle comprising awet grinding and a Wet classifying step coupled in a circuit, in whichmethod discharge from the secondary grinding step is returned to theprimary classifying step and in which portions of the grinding load areshiftable between said grinding stages, for instance, from the secnd tothe first grinding stage, by effecting a control of size separation insaid primary classifying stage.

15. .A stagewise wet grinding system comprising a first grinding stageand a first classifying stage; a second grinding stage, and a secondclassifying stage; means for introducing new feed material into thesystem by way of the first grinding stage; means fortransferring-overflow from the first to the second classifying stage;means for re-cycling classified material from the first classifyingstage to the one of said grinding stages; means for recycling classifiedmaterial from the second classifying stage to the other of said grindingstages; and means for transferring material from the discharge of bothgrinding stages to the first classifying stage.

16. A method of stagewis'e wet grinding, which comprises grindingmaterial to be treated in parallel operation in a first and in a secondgrinding stage; introducing new feed material into the system by way ofthe first grinding stage; transferring material from the discharge ofboth grinding stages to a first wet classifying stage; recyclingclassified material from the first classiying stage to the one of saidgrinding stages while transferring the overflow from that classifyingstage to a second wet classifying stage;

- and means for recycling classified material from said secondclassifying stage to the other of said grinding stages.

JAMES V. SLADE.

